With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 50%. In recent years, an information recording capacity exceeding 320 gigabytes per platter has been desired for a magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like. To fulfill such demands, an information recording density exceeding 500 gigabits per square inch is desired to be achieved.
Important factors for increasing recording density of the perpendicular magnetic disk include, for example, an improvement in TPI (Tracks per Inch) by narrowing the track width, ensuring electromagnetic conversion characteristics, such as a Signal-to-Noise Ratio (SNR) and an overwrite (OW) characteristic at the time of improving BPI (Bits per Inch), and further ensuring heat fluctuation resistance with recording bits decreased due to the above. Among these, an increase in SNR in a high recording density condition is important.
In a magnetic layer of a granular structure (hereinafter referred to as a granular magnetic layer), which goes mainstream in recent years, a nonmagnetic substance having an oxide as a main component is segregated around magnetic particles having a CoCrPt alloy growing in a columnar shape to form a grain boundary part. In this structure, since the magnetic particles are separated from each other, noise is reduced, and this is effective for a high SNR. An important factor to further increase the SNR is to microfabricate crystal particles, equalize particle diameters (which are collectively referred to as “particle diameter control), isolate the crystal particles, and improve crystal orientation. Co takes a hcp structure (a hexagonal close-packed crystal lattice structure), and a c-axis direction (an axial direction of a hexagonal column as a crystal lattice) serves as an axis of easy magnetization. Therefore, by orienting the c axis of each of more crystals in a more perpendicular direction, noise is reduced and signals become strong, which can cause an increase in SNR as a synergy effect.
Meanwhile, when a film of crystals is formed on crystals by sputtering, crystal orientation tends to be improved as the film thickness becomes thicker due to epitaxial growth. Thus, to microfabricate crystal particles, equalize particle diameters, and enhance crystal orientation of a granular magnetic layer from an initial growth stage, conventionally performed are processes of forming a film of a ground layer (which is also called an intermediate layer) made of Ru, which is a metal with a hcp structure, and then forming a film of the granular magnetic layer on the ground layer.
While Ti, V, Zr, and Hf are known as examples of a material for use in the ground layer, as described in Patent Document 1, Ru (ruthenium) currently goes mainstream. The reason for this is such that Ru takes a hcp structure, can effectively improve perpendicular orientation of the axis of easy magnetization of a granular magnetic layer having Co (cobalt) as a main component, and can achieve a high SNR.
It is also known that the function of the film of the ground layer varies by the pressure of an atmospheric gas in a film formation process even if the material is the same. For example, Patent Document 2 suggests the structure having, as a ground film of a perpendicular magnetic layer, a layer containing ruthenium with its film being formed under a high-pressure argon atmosphere (on the order of 6 Pa to 10 Pa) and a layer containing ruthenium with its film being formed under a low-pressure argon atmosphere (in the neighborhood of 1 Pa). With this, according to the document, an effect is achieved in which the magnetic layer becomes highly oriented in the low-pressure Ru layer and the magnetic layer have fine particles in the high-pressure Ru layer.
Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2008-140460) also suggests a technology in which, by adding oxygen or an oxide to a ground layer made of Ru, the added substance is segregated between Ru to isolate particles of Ru and promote isolation of magnetic particles of a granular magnetic layer formed as a film on the ground layer.